This invention relates to welded aluminum alloy members and more particularly it relates to a method for improving weld strength in the weld zone and the heat affected zone of the weld.
Welding aluminum alloys normally involves generation or application of heat to melt or plasticize the aluminum alloy members being welded. Welding has a detrimental effect on the properties of alloy members being joined by decreasing strength in the weld and in the zone adjacent the weld referred to herein as the heat affected zone. This is particularly true for the precipitation hardened or strengthened alloys referred to by the Aluminum Association alloys such as the AA2xxx, AA6xxx and AA7xxx alloy series. In these alloys, heat generated during welding is high enough to dissolve precipitation strengthening phases such as MgZn2 (η-phase), Mg5Al3, Mg3Zn3Al2 (T-phase), Mg4Zn7Al, Al2Cu, Mg(Al, Cu, Zn)2, CuMgAl2 (S-phase), LiAl, LiMgAl2 and Al3Li, depending on the alloy. After the welding process, the phases start to naturally re-precipitate at a very slow rate and the re-precipitation can last for ten or fifteen years, resulting in an unstable weld. Such welds also exhibit poor corrosion resistance.
Welding is also detrimental in the adjacent heat affected zone because of the heat generated during the welding procedure. That is, the temperatures in the heat affected zone are lower than in the weld but are sufficiently high to cause precipitation and coarsening of these phases. This results in overaging in the heat affected zone and a reduction in strength. Thus, the forming of the weld results in a decrease in strength and corrosion properties in the structure.
Past attempts at solving the problem of decreased strength included increasing the thickness of the metal in the weld zone. However, this has the problem of adding more weight which is undesirable for aircraft use. Further, increased thickness did not solve the problem of instability of the weld and did not prevent overaging in the heat affected zone.
These problems arise with different welding techniques such as friction welding, friction stir welding or laser welding. In friction welding, relative movement of the parts to be welded is employed to generate heat for joining. In friction stir welding, a rotating non-consumable tool is used to generate heat in the members to be joined. Laser welding employs a laser beam to melt a small area of each of the parts to be joined. In these welding techniques, the stirred or melted portion flows together and solidifies to provide a welded structure.
It can be seen that there is a great need for a method of welding which eliminates or minimizes these problems to produce a weld and heat affected zone having improved properties.
Different approaches have been used in an attempt to solve these problems. For example, U.S. Pat. No. 6,168,067 discloses a method for reducing material property degradation during friction stir welding. More specifically, the method includes the steps of solution heat treating first and second structural members at a first predetermined temperature schedule. The first and second structural members are then quenched to a predetermined temperature at which the structural members are in a non-equilibrium state and have an incomplete temper. The first structural member is then positioned adjacent to the second structural member, thereby defining an interface therebetween. Thereafter, the first and second structural members are joined to form a structural assembly by friction stir welding the material along the interface prior to precipitation heat treating the structural assembly. The structural assembly is then aged, such as by precipitation heat treating, at a second predetermined temperature schedule to stabilize the material properties of the resulting structural assembly, thereby completing the temper of the material. The method requires fewer manufacturing steps than conventional techniques for friction stir weld precipitation-hardened parent materials. In addition, the method minimizes the degradation of the material properties during friction stir welding, and produces a structural assembly with improved strength, hardness, and corrosion resistance, as well as dimensional quality.
U.S. Pat. No. 5,248,077 discloses that friction welding is preformed by linear or non-rotating orbital motion in which both parts to be joined by friction welding are moved in identical paths, out of phase to produce friction until welding temperature is attained, and then in phase, with the parts registered in alignment, during the bonding phase. Because the registered, in phase control of the mechanism is attainable with great certainty and rapidity, friction welding with a far higher precision and accuracy of alignment is attained. The high precision and accuracy of the technique permits application of friction welding to fabrication of parts, such as turbine engines, not usually made by such techniques. A preferred apparatus for practice of the invention involves an opposed pair of orbital tables on which parts to be joined are mounted and fixed in place by mounts. The parts are aligned and registered and drives then cause the parts to orbit while they are pressed together by pressure means, until friction heats the joint surfaces to welding temperatures.
U.S. Pat. No. 5,507,888 discloses methods for making an aluminum alloy bicycle frame and for making tubes for such frames including use of an aluminum alloy containing about 0.5 to 1.3% magnesium, about 0.4 to 1.2% silicon, and about 0.6 to 1.2% copper and preferred practices for making extruded and drawn tubing of the alloy and making bicycle frames from the tubing. The preferred practices include extrusion temperature control and other aspects of extrusion and drawing. The tubes are welded by MIG or TIG welding procedures.
U.S. Pat. No. 5,897,047 discloses a method of connecting an aluminum part with a steel part using a friction welding technique. The aluminum part is heat treated such as T6 treatment to raise its hardness. After that, the aluminum part is friction welded to the steel part. The steel part may also be heat treated to lower its hardness instead of or in addition to hardening of the aluminum part. A third part which has an intermediate hardness may be placed between the aluminum part and the steel part, and the aluminum part may be coupled with the steel part via the third part.
U.S. Pat. No. 6,333,484 discloses a process for welding a nickel or cobalt based superalloy article to minimize cracking by preheating the entire weld area to a maximum ductility temperature range, maintaining such temperature during welding and solidification of the weld, raising the temperature for stress relief of the superalloy, then cooling at a rate effective to minimize gamma prime precipitation.
PCT Application WO 98/45080 discloses a friction stir welding method according to which the workpieces to be welded are positioned on a work-table and by means of clamping means clamped to one another and/or to the work-table during the welding. A rotating welding means is arranged to move along a joint between the workpieces while being pressed against said workpieces during the welding. Additional heat is supplied to the joint prior to and/or during the welding operation, in excess of the frictional heat generated in the joint from the rotation of the welding means and of any other heat that may be supplied to the joint in any other manner by the welding means. The invention likewise concerns an apparatus for friction stir welding, comprising a heating element for supply of additional heat to the joint prior to and/or during the welding operation, in excess of the frictional heat generated in the joint from the rotation of the welding means and of any other heat that may be supplied to the joint in any other manner by the welding means.
The present invention provides an improved welded assembly and provides a novel process for welding of precipitation strengthened aluminum alloys having improved weld strength as well as improved strength in the heat affected zone adjacent the weld. Also, the novel process results in improved corrosion properties in the weld and heat affected zone.